Power converters are, for example, used in wind turbines for converting a variable frequency AC power provided by the wind turbine generator into a nominally fixed frequency AC power to be fed to a grid. Such power converters typically comprise a rectifier or active rectifier for converting the variable frequency AC power into a DC power and an inverter for converting the DC power into the fixed frequency AC power. Both the rectifier and the inverter typically comprise two DC voltage terminals and three AC voltage terminals. In case of the rectifier the AC voltage terminals are connected to the generator output terminals providing, e.g., a three phase AC power and the DC terminals are connected to a DC link between the rectifier, and the inverter. The inverter also comprises two DC terminals connected to the DC link. In addition, it comprises three AC terminals connected to a grid via appropriate filtering circuits as may be required. Active rectifier and inverter may be formed from the same circuit components but with different power flow (AC to DC in case of the active rectifier, DC to AC in case of the inverter).
Other applications of power converters comprise, i.e., conversion of a fixed frequency AC power to a variable frequency AC power, for example for controlling the rotational speed and/or torque of an electric motor.
Typical configurations of a rectifier and an inverter, both commonly referred to as power converters in the following, comprise a series of at least two active switching devices connected between the upper voltage level of the DC link and the lower voltage level of the DC link and a node between both active switching devices which is connected to one of the AC terminals. Such a design is known as half bridge, or phase. The same structure is present for all other AC voltage terminals of the power converter so that a power converter for a three phase AC power has three half bridges each comprising at least two active switching devices. The structure with two active switching devices in a given half-bridge is known as a two-level converter in that by appropriate control the output voltage seen at the centre phase terminal can be either the upper voltage level of the DC link or the lower voltage level of the de link.
Power conversion by use of the active switching devices is typically done in the following way:
In case of converting DC power to AC power each AC voltage terminal is connected through the active switching devices to the high DC level and the low DC level in an alternating fashion. By introducing a phase shift between the command signals defining the output of each AC terminal a polyphase AC power, for example a three phase AC power, can be established. The AC power may be designed to be a balanced AC power, e.g. a three phase AC power in which the three phase currents always sum up to zero.
In case of converting AC power into DC power the active switching devices are switched for each AC input terminal such that the terminal is connected to the upper DC voltage terminal or the lower DC voltage terminal.
For both modes of power conversion, switching the active switching devices is typically performed on the basis of a pulse width modulation scheme in which time and duration of an active switching device being ON, i.e. conductive, or OFF, i.e. non-conductive, is defined by high level or low level switching pulses, respectively. Other schemes for determining the switching of the active devices is equally valid including direct power control, direct current control, direct torque control or equivalent.
Sometimes, two or more half bridges are connected in parallel or in series to an AC terminal, in particular in power converters having a high power rating. In case, for example, three half bridges each comprising two switching devices would be connected in parallel in a three phase converter the whole power converter would comprise 12 active switching devices (three times two times two). Typically, a power converter comprising parallel or serial half bridges is organized in the form of converter modules each of which comprises one half bridge for each of the AC terminals. These power modules are connected in parallel or in series to form the power converter. In particular, for high power rated power converters for industrial drives and renewable energy applications, it is a standard technique to construct these converters from multiple converter modules operating in parallel or in series to achieve the necessary voltage, current and power rating.
It is desirable to use a central controller for all converter modules in a power converter. A difficulty thereby is to build a communication system between the central controller, which could also be considered as a main control system or a real time computer, running the control algorithms and the distributed converter modules of the power converter. The main requirements of such of a communication systems are to transmit the switching control signals to the converter modules with a high degree of timing precision, a high degree of edge resolution of the switching states in a given pulse width modulation scheme, a low latency response to error conditions, and tolerance to single bit errors. Furthermore, it should comprise viable and affordable physical media for interconnections between the central controller and the converter modules. In addition, the communication system should preferably be able to transmit current, voltage and other analogue feedback signals and logical status signals, i.e. digital signals, from the converter modules to the central controller.
Document WO 2009/087063 A1 discloses a power converter with distributed cell control in which a central control unit transmits a reference AC voltage and a switching carrier signal to controller subunits where each subunit controls the switching of power electronic switches according to a pulse width modulation pattern so that each time the switching carrier signal crosses the reference AC voltage either a high DC voltage or a low DC voltage is applied to output terminals of the corresponding converter submodule. Hence, the actual switching signals for the active switching devices are determined at a local level rather than by the central controller. It is, however, desirable to calculate all switching states at the central controller. This, however, means that in case of a three phase AC converter using a two-level half bridge structure for each converter module at least twelve switching states need to be transmitted within a given PWM period (a switching state representing the “on-state” and a switching state representing the “off-state” for each of the six active switching devices).
There exist two approaches to transmit the switching states from a central controller to the converter module in the state of the art. The first one is to use a parallel connection from the central controller to each of the distributed converter modules. Such a parallel connection would typically be an electrical ribbon cable. This parallel connection carries individual copper communication channels for each piece of data, e.g. six ways for the switching control signals for the active switching devices, three ways for the three current feedback signals, three ways for three voltage feedback signals, etc. This kind of parallel connection has some drawbacks, for example, the amount of circuitry that is needed at both ends of the communication link. Moreover, the information transferred over the communication link is fixed by the circuitry at both ends so that this type of connection is inflexible. In addition, the distance which can be realised with a parallel connection channel is usually restricted.
The second state of the art approach to a link comprises a high-performance serial link. The connection topology for such a link is a daisy chain system. However, this means that the data payload is comparatively large as the single connection from/to the central controller has to carry information for all the distributed converter modules, and the message interval therefore has to be large to keep the bandwidth requirement of the communication channel within a practical and affordable range. Typically, the message interval is once per pulse width modulation period. This means that emergency conditions have to be catered for by a separate connection between the converter modules. Additionally, certain control modes which have a CPU calculation period less than the cycle time of the communication link are not realisable in this system.